Method for coating metal surfaces

ABSTRACT

A method for coating a light metal alloy component to form a protective layer comprising Sn. First, a surface of the light metal alloy component is cleaned and passivated. A layer comprising Zn is formed on the surface, and a layer comprising Sn is deposited. An intermediate layer is preferably deposited between the Zn-containing layer and the Sn containing layer. The Sn-containing layer may additionally be varnished.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a method for coating a surface of alight metal alloy component. Light metal alloys include, but are notlimited to, alloys that contain Al and/or Mg in an amount thatcontributes considerably to determining the chemical properties of thesurface. Because of their low specific gravity, light metal alloys areof great interest for many different applications in which both highmechanical stability and the total weight of the component areimportant, for example, in aircraft construction, motor vehicles, orhousings for high quality devices. Also, light metal alloy frame partslend stability to portable metal telephones while burdening the user aslittle as possible. However, light metal alloys are sensitive tooxidation, so they require surface treatment to avoid corrosionproblems. Typical treatment methods have the disadvantages of satisfyingtechnical requirements only to a limited extent, being very costly, orunduly restricting the size or geometry of the parts that may betreated. Also, such surface treatments may have a negative effect on theappearance of light metal alloy components.

SUMMARY OF THE INVENTION

[0002] Among the several objects of the present invention, therefore,may be noted the provision of a method for coating a light metal alloywhich is effective for corrosion protection. A further object of thepresent invention is the provision of a method of coating a light metalalloy which is capable of coating a wide variety of surfaces with regardto size and shape. A further object of the present invention is theprovision of a protective coating for a light metal alloy which has anappealing appearance.

[0003] Briefly therefore, the present invention is directed to a methodfor coating a metallic surface which comprises cleaning and passivatinga surface of a light metal alloy component and forming a first layercomprising Zn on the surface. A second layer comprising Sn is formedsuch that the first layer is located between the surface and the secondlayer. The present invention is further directed to a coating for alight metal alloy comprising a first layer which comprises Zn and asecond layer which comprises Sn, wherein the first layer is locatedbetween the light metal alloy surface and the second layer.

[0004] Other objects and features of the present invention will be inpart apparent and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWING

[0005]FIG. 1 is a view of the internal surface of a diecast Mg alloyAZ91 chassis for a mobile telephone housing.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0006] In metallurgy and materials science, light-metal alloys areunderstood to encompass a variety of metal mixtures comprising “lightmetals,” such as Al, Be, Mg, and Ti. The most common light-metal alloyscontain Al or Mg. Preferably, the method according to this invention isperformed on light-metal alloys with a relatively high Al content orthose with a relatively high Mg content.

[0007] A Sn-containing layer enables reliable finishing of a light metalalloy surface. It has been discovered that a protective layer comprisingSn adheres well to a light metal alloy surface which has first beencleaned and passivated and then coated with a Zn-containing layer.

[0008] Cleaning and Passivating

[0009] An alkaline degreasing of the light metal alloy surface is auseful initial step for cleaning and passivating. The degreased surfaceis preferably treated with a solution which is acidic or which comprisesthe salt of an acid to perform some etching of the light metal alloysurface and which also performs an oxidative passivation. The termoxidation is generally understood here to mean a valence electrontransition and in particular implies the formation of oxides like Al₂O₃and fluorides like MgF₂.

[0010] In one embodiment, wherein the alloy comprises Mg, preferablyhaving an Mg fraction of at least about 50 weight %, especially at leastabout 80 weight %, the cleaning and passivating may be performed in twosteps. First, the surface is treated with relatively weak acidicsolution having a pH from about 3 to about 5, preferably about 4. Then,the surface is treated with a relatively strong acidic solutioncomprising fluoride ions and having a pH in the range of about 0.5 toabout 2, preferably about. During the etching of the surface, thefluoride ions form a passivating layer comprising MgF₂. The weak acidicsolution may comprise, for example, carboxylic, citric acid, malic acid,oxalic acid, lactic acid, a pyrophosphate, and combinations thereof. Thestrong acidic solution may comprise, for example, a mixture ofphosphoric acid and ammonium bifluoride.

[0011] In another embodiment, wherein the alloy comprises Al, preferablyhaving an Al fraction of at least 60 weight %, especially at least 80weight %, the treatment is preferably performed with a highly oxidizingsolution that simultaneously etches the surface and produces apassivating layer comprising Al₂O₃. Examples of strongly oxidizingsolutions are nitric acid, peroxomonosulfuric acid, and a potassiumpersulfate solution.

[0012] Cleaning and passivating may also be performed anodically with asolution comprising phosphoric acid and an alcohol as described inco-pending U.S. application Ser. No. 10/176,308, filed on Jun. 20, 2002,which is herein incorporated by reference in its entirety. Such cleaningprovides effective degreasing and etching of the surface, and the anodicoperation allows for flexible optimization via parameters such as anodiccurrent density, voltage, and the like.

[0013] The alcohol may be, for example, methanol, ethanol, propanol,butanol, a polyhydric alcohol, or derivatives, such as isopropanol.Diols, polyethers and other alcohols are also useful, as are mixtures ofalcohols. Preferably the alcohol comprises butanol or isopropanol.

[0014] Preferably, fluoride ions are used as described elsewhere hereinto passivate the surface of a Mn-containing alloy. The fluoride ions maybe in the form of, for example, ammonium bifluoride, an alkali fluoride,hydrofluoric acid, as well as other forms. The fluoride ions may be in asolution with the phosphoric acid, with the alcohol, or with thephosphoric acid and the alcohol. In a multi-step cleaning andpassivation process, the step performed with fluoride ions is preferablyperformed last.

[0015] Treatments with fluoride ions, especially the two-step acidtreatments and treatments with phosphoric acid and alcohol, are alsouseful when the light metal alloy comprises no or little Mg, butcomprises Si, preferably at least about 0.1 weight %, and morepreferably at least about 0.5, 1 or 2 weight % or higher. The fluorideion concentration in this case is dependent upon the Si concentration.

[0016] The surface treatment with phosphoric acid, alcohol, and fluorideions may further comprise an alkaline rinse step, preferably with anaqueous solution having a pH of at least about 10. However, an alkalinerinse step is less advantageous for a passivation surface dominated byAl₂O₃.

[0017] Anodic treatment of alloys comprising Al preferably employstreatment of the surface with an aqueous oxidation agent such as apersulfate solution or a solution of peroxomonosulfuric acid (Caro'sacid). Oxidation is preferably performed after any fluoride treatment.An aqueous oxidation step at a pH of less than about 6 may beproblematic if the alloy also has a high Mg fraction because thefluoride passivation can be damaged.

[0018] Useful anodic current densities have a lower limit of about 10,30 or 50 A/m² and an upper limit of about 1000 A/m². Preferably, thelight metal alloy surface is cleaned and passivated at a temperature offrom about 10° C. to about 40° C. The solution used in the anodiccleaning steps comprises phosphoric acid in an amount which preferablyranges from about 30 to about 90 percent of the solution on a volumetricbasis. Within this range of volume fractions, the phosphoric acid canmeasure from about 50 to about 95 percent H₃PO₄ by weight. The solutionfurther comprises an alcohol and, optionally, fluoride ions. Usefulfluoride solutions have a fluoride content of about 0.1, 0.3 or 0.5weight % as a lower limit and about 30, 20 or 10 weight % as an upperlimit.

[0019] Coating

[0020] After the cleaning and passivating pretreatments, a layercomprising Zn and an layer comprising Sb is applied to the light metalalloy surface. Preferably, the layer comprising Zn chemically metalplated. This metal plating can additionally contain the metals Cu and/orNi. Preferably, the layer comprising Sn is electrolytically coated. Theamount of Sn in this electrolytic layer is preferably at least about 40weight %, more preferably at least about 50 weight %. This layer mayalso contain, for example, Zn, Bi and/or Pb in addition to Sb, in orderto improve the corrosion properties.

[0021] The Zn-containing layer is preferably electrolytically coatedwith an intermediate layer to protect the Zn-containing layer fromdamage by subsequent coating steps, for example, the electrolyticcoating with the Sn-containing layer. The intermediate layer maycomprise Cu and/or Ni. The specific process chosen for the intermediatecoating is matched to the stability of the Zn-containing metal plating.The intermediate layer is preferably coated at a pH from about 7 toabout 10 because the Zn-containing layer can be damaged by processeswhich are too acidic as well as process which are too alkaline.Processes at pH which may damage the Zn-containing layer may bedesirable or unavoidable in the production of the Sn-containing layer.Preferred layer thicknesses for the intermediate layer lie are betweenabout 5 and about 10: m. Preferred layer thicknesses for theSn-containing layer are between about 5 and about 10: m.

[0022] The method according to the present invention provides stable andpermanent electrolytic coatings on light metal alloy surfaces. Since themethod can be carried out with wet chemical and electrolytic processsteps, it is very flexible with regard to the usable part sizes andgeometries and incidentally can be carried out inexpensively on a largescale. In the above-described procedures, a metallic conductive surfaceis achieved, which is desirable for many applications.

[0023] However, a particular appeal of the invention lies in the factthat a varnish may additionally be deposited on the Sn-containingelectrolytic layer. This provides far-reaching freedom with regard tothe visual design of the surface. For example, the varnish can becolored to be opaque or transparent. In this way many different kinds ofdecoration effects can be achieved. It can also have structures, forexample, surface spattering, which can be applied with conventionalvarnishing machines in a standard way, in order to give the treated partan individual visual and tactile appearance. Furthermore, the varnishedsurface is characteristically electrically insulating, which can bedesirable, depending on the application. Finally, better corrosionprotection may be afforded by the varnish layer. Preferably, atwo-component varnish is used. One-component varnishes are useful, butthey generally have poorer technical performance.

[0024] The adhesion of the varnish is improved if the Sn-containinglayer is passivated prior to applying the varnish. Passivation ispreferably performed by alkaline anodic oxidation, for example, with asolution that contains phosphates and/or carbonates. This alkalineanodic oxidation can be supplemented by a subsequent cathodic treatmentin a solution of hexavalent chromium, for example, chromic acid. Thisresults in a coating of the surface with trivalent chromium. From thestandpoint of health and environment the use of hexavalent chromium is,however, problematic (although not for the product itself), due to whichvarnishing the electrolytic surface that has only been pretreated byalkaline anodic oxidation is preferable.

[0025] In addition to the advantages already described, the varnishedsurface may be subsequently subsequently treated to return conductivityto partial areas. This can be useful, for example, in order to applyelectrical contacts to the coated component at specific sites, but wherethe component is supposed to remain insulated or coated with varnish forvisual appeal or for protection against chemical and mechanical stress.

[0026] In one embodiment, a laser is used to chip off or evaporate thevarnish at selected parts of the surface and through a remelting bringsthese areas to a metallic conductivity. An exposed Sn-containing layerprovides good electrical conductivity and provides stability for theregion from which the varnish has been removed. Incidentally, lasertreatment can also be advantageous in the case of nonvarnished partscoated in accordance with the invention in order to give someimprovement to the already existing surface conductivity. Finally, thelaser treatment can also be used if the surface treated in accordancewith the invention is provided with other or additional insulatinglayers, for instance with sputtered oxides, nitrides and the like. It ispreferable to apply a flowable metallically conductive substance, forexample, an adhesive or another plastic-based hardening substance thatcontains metallically conductive particles, onto the laser-bombardedregions of the surface within a few hours or a few days. Silverparticles or silver-coated particles are useful. The laser bombardmentis preferably carried out at least two times in order to limit thethermal stress on the surface, and the laser bombardment be performed inan atmosphere of air with a conventional apparatus. An Nd:YAG laser, forexample, a 90 W laser, has proven to be suitable. The process of lasertreatment is described in detail in European Patent Application No.01124434.0, filed on Oct. 11, 2001 and titled “Producing metallicallyconductive surface regions on coated light metal alloys” and filed onOct. 11, 2001, which is herein incorporated by reference in itsentirety.

[0027]FIG. 1 is an internal view of a diecast Mg alloy AZ91 frame part1, or so-called chassis, of a mobile telephone housing. Frame part 1 isglued to other metallic or metallically coated housing parts along astrip 2. It is important that the frame part 1 have good long-termsurface adhesion and a high grade appearance. Through frequent contactwith hands and the resulting simultaneous effect of salts, weak acidsand moisture, and also through the effects of weather and othercircumstances in long-term use, a surface of frame part 1, not shown,can become unsightly if there is insufficient coating. Furthermore,corrosion of an inner surface, not shown, could lead to the formation ofparticles and thus to failure of electronic components.

[0028] In the gluing of the parts of a mobile phone, it is additionallyimportant that the glued parts are bonded to each other while retaininggood electrical conductivity in order to provide electromagneticshielding for the telephone. Thus, a stable coating of the frame part 1must provide good electrical surface conductivity along the strip 2 towhich glue is applied. The same is true for flat parts of the indicatedsupport dome 3 for a circuitboard, which likewise become conductivebecause of the necessary mass connection. Other details of the framepart 1 are not important for understanding of the invention.

[0029] The following examples illustrate the invention.

EXAMPLE

[0030] The frame part 1 is first conventionally degreased by alkalitreatment and treated at pH 4 in a solution with citric acid andpyrophosphate, followed by passivation at pH 1 in a strongly acidicsolution with phosphoric acid and ammonium bifluoride.

[0031] A chemical conversion layer of Zn and Cu is applied to thecleaned and passivated surface. Onto this layer a 7: m thick Cu layer isthen be deposited by conventional electrolysis. Then an electrolyticlayer of Sn and Zn is deposited on the electrolytic layer of Cu. Theweight ratio of Zn:Sn is 70:30. The layer thickness is 8: m.

[0032] This still electrolytically conductive surface is now preparedfor varnishing with an alkaline anodic oxidation in a phosphatesolution. A treatment with hexavalent chromium is not employed. Instead,a commercial two-component varnish is applied directly onto the anodizedsurface and hardened. The surface of the Mg diecast frame part 1 has thefinal visual and technical quality so that it can be varnished with anabsolutely transparent color, so that an attractive appearance resultsfrom the metal shining through the varnish.

[0033] This surface is then treated on the indicated strip 2 and supportdomes 3 with a commercial Nd:YAG laser. This laser is Q switched and hasa power of 90 W at a lamp current of about 32 A. The strip 2 and domes 3are traced two times, precisely setting point next to point.

[0034] Empirically, the point spacing, point size and energy per pointcan be determined so that a continuous strip of sufficient widthresults. The strip width should not be too small, in order to optimizethe electrical contact resistance with the other part of the housing. Onthe other hand, the strip width should not be too great and should becompletely covered by the subsequently applied bead of adhesive (1 mmwide in this example). Also, the coupled energy per shot should not beunnecessarily high, in order to avoid heating that is too great atgreater depths. By two-fold bombardment the energy per shot can be keptsmall. In this case 15 W/mm² is used per shot. The laser feed rate inthis case is 400 mm/sec.

[0035] Then a bead of silicone glue mixed with silver particles can beapplied to the thus remetallized surface regions 2 and 3, so that anelectrically conductive glueing to another housing part, not shown, cantake place. This other housing part is likewise metallic or metallicallycoated and is glued so that it obtains electrical contact to theadhesive. In this way an electrical contact to the adhesive is obtained,and a tight and electrically shielded housing can be produced.

[0036] In view of the above, it will be seen that the several objects ofthe invention are achieved.

[0037] As various changes could be made in the above material andprocesses without departing from the scope of the invention, it isintended that all matter contained in the above description beinterpreted as illustrative and not in a limiting sense.

What is claimed is:
 1. A method for coating a metallic surfacecomprising: cleaning and passivating a surface of a light metal alloycomponent; forming a first layer on the cleaned and passivated surfacewherein the first layer comprises Zn; and forming a second layer whichcomprises Sn such that the first layer is located between the surface ofthe light metal alloy component and the second layer.
 2. The methodaccording to claim 1 wherein the cleaning and passivating stepcomprises: alkaline degreasing the surface of the light metal alloycomponent; and performing an acid treatment of the surface tooxidatively produce a passivation layer on the surface, the acidtreatment comprising contacting the surface with a solution which isselected from a group consisting of an acidic solution, a solutioncomprising the salt of an acid, and combinations thereof.
 3. The methodaccording to claim 2 wherein the light metal alloy comprises Mg.
 4. Themethod according to claim 3 wherein the acid treatment step comprises:contacting the surface of the light metal alloy component with a weakacidic solution; and contacting the surface with a strong acidicsolution which comprises fluoride ions.
 5. The method according to claim4 wherein the light metal alloy has a Mg composition of at least about50 weight %.
 6. The method according to claim 5 wherein the weak acidicsolution has a pH of from about 3 to about
 5. 7. The method according toclaim 5 wherein the weak acidic solution comprises a carboxylic acid anda pyrophosphate.
 8. The method according to claim 5 wherein the strongacidic acid solution has a pH of from about 0.5 to about
 2. 9. Themethod according to claim 5 wherein the strong acidic acid solutioncomprises phosphoric acid and ammonium bifluoride.
 10. The methodaccording to claim 5 wherein the weak acidic solution has a pH of fromabout 3 to about 5 and the strong acidic acid solution has a pH of fromabout 0.5 to about
 2. 11. The method according to claim 5 wherein theweak acidic solution comprises a carboxylic acid and a pyrophosphate andwherein the strong acidic acid solution comprises phosphoric acid andammonium bifluoride.
 12. The method according to claim 5 wherein theweak acidic solution has a pH of from about 3 to about 5 and comprises acarboxylic acid and a pyrophosphate and wherein the strong acidic acidsolution has a pH of from about 0.5 to about 2 and comprises phosphoricacid and ammonium bifluoride.
 13. The method according to claim 2wherein the light metal alloy comprises Al.
 14. The method according toclaim 13 wherein the acid treatment step comprises contacting thesurface of the light metal alloy component with a strong oxidizingsolution.
 15. The method according to claim 14 wherein the light metalalloy has an Al composition of at least about 60 weight %.
 16. Themethod according to claim 15 wherein the strong oxidizing solutioncomprises an oxidizer selected from the group consisting of nitric acid,peroxomonosulfuric acid, and a persulfate solution.
 17. The methodaccording to claim 1 wherein the cleaning and passivating of the surfaceof the light metal alloy component comprises: anodically connecting thesurface to an electrical source; and contacting the surface with asolution which comprises phosphoric acid and an alcohol.
 18. The methodaccording to claim 17 wherein the light metal alloy comprises a metalselected from the group consisting of Mg, Si, and combinations thereof.19. The method according to claim 18 wherein the light metal alloy has aMg composition of at least about 50 weight %.
 20. The method accordingto claim 18 wherein the light metal alloy has a Si composition of atleast about 0.1 weight %.
 21. The method according to claim 18 whereinthe cleaning and passivating step comprises contacting the anodicallyconnected surface with a solution that comprises phosphoric acid andfluoride ions.
 22. The method according to claim 17 wherein the lightmetal alloy comprises Al.
 23. The method according to claim 22 whereinthe light metal alloy has an Al composition of at least about 60 weight%.
 24. The method according to claim 22 wherein the cleaning andpassivating step comprises contacting the surface of the light metalalloy component with an aqueous oxidation agent.
 25. The methodaccording to claim 1 wherein the first layer is formed by chemical metalplating.
 26. The method according to claim 1 wherein the first layerfurther comprises a metal selected from the group consisting of Cu, Ni,and combinations thereof.
 27. The method according to claim 1 whereinthe second layer is formed by electrolytic deposition.
 28. The methodaccording to claim 1 wherein the second layer further comprises a metalselected from the group consisting of Zn, Bi, Pb, and combinationsthereof.
 29. The method according to claim 1 further comprising formingan intermediate layer such that the intermediate layer is locatedbetween the first layer and the second layer.
 30. The method accordingto claim 29 wherein the intermediate layer is formed by electrolyticdeposition.
 31. The method according to claim 29 wherein theintermediate layer comprises a metal selected from the group consistingof Cu, Ni, and combinations thereof.
 32. The method according to claim 1further comprising depositing varnish layer on the second layer whereinthe varnish layer comprises a varnish.
 33. The method according to claim32 wherein the varnish is a two-component varnish.
 34. The methodaccording to claim 32 further comprising performing a passivatingtreatment on the second layer prior to depositing the varnish layer. 35.The method according to claim 34 wherein the passivating treatmentcomprises an alkaline anodic oxidation.
 36. The method according toclaim 35 wherein the passivating treatment comprises contacting thesecond layer with a solution comprising a compounds selected from thegroup consisting of phosphates, carbonates, and combinations thereof.37. The method according to claim 35 wherein the passivating treatmentfurther comprises a cathodic treatment wherein the second layer iscontacted with a solution comprising hexavalent chromium ions.
 38. Themethod according to claim 32 further comprising removing a portion ofthe varnish layer to expose a portion of the second layer.
 39. Themethod according to claim 38 wherein the portion of the varnish layer isremoved by bombarding the varnish layer with a laser beam.
 40. Themethod according to claim 39 wherein the varnish layer is bombarded bythe laser beam at least twice to remove the portion of the varnishlayer.
 41. A method for coating a metallic surface comprising: alkalinedegreasing the surface of a light metal alloy component, wherein thelight metal alloy component has a Mg composition of at least about 50weight %; performing an acid treatment of the surface to oxidativelyproduce a passivation layer on the surface, the acid treatmentcomprising: contacting the surface of the light metal alloy componentwith a weak acidic solution having a pH of from about 3 to about 5; andcontacting the surface with a strong acidic solution which has a pH offrom about 0.5 to about 2 which comprises fluoride ions; forming a firstlayer on the cleaned and passivated surface wherein the first layercomprises Zn; and forming a second layer which comprises Sn such thatthe first layer is located between the surface of the light metal alloycomponent and the second layer.
 42. The method according to claim 41further comprising forming an intermediate layer such that theintermediate layer is located between the first layer and the secondlayer.
 43. The method according to claim 42 wherein the intermediatelayer is formed by electrolytic deposition.
 44. The method according toclaim 42 wherein the intermediate layer comprises a metal selected fromthe group consisting of Cu, Ni, and combinations thereof.
 45. A methodfor coating a metallic surface comprising: contacting a surface of alight metal alloy component with an aqueous oxidation agent, wherein thelight metal alloy component has an Al composition of at least about 60weight %; forming a first layer on the surface wherein the first layercomprises Zn; and forming a second layer which comprises Sn such thatthe first layer is located between the surface of the light metal alloycomponent and the second layer.
 46. The method according to claim 45further comprising forming an intermediate layer such that theintermediate layer is located between the first layer and the secondlayer.
 47. The method according to claim 46 wherein the intermediatelayer is formed by electrolytic deposition.
 48. The method according toclaim 46 wherein the intermediate layer comprises a metal selected fromthe group consisting of Cu, Ni, and combinations thereof.
 49. A coatingfor a light metal alloy comprising: a first layer comprising Zn whereinthe first layer is deposited on a surface of the light metal alloycomponent; and a second layer comprising Sn, wherein the first layer islocated between the surface of the light metal alloy component and thesecond layer.
 50. The coating according to claim 49 wherein the lightmetal alloy comprises Mg.
 51. The coating according to claim 49 whereinthe light metal alloy comprises Al.
 52. The coating according to claim49 wherein the first layer is formed by is formed by chemical metalplating.
 53. The coating according to claim 49 wherein the first layerfurther comprises a metal selected from the group consisting of Cu, Ni,and combinations thereof.
 54. The coating according to claim 49 whereinthe second layer is formed by electrolytic deposition.
 55. The coatingaccording to claim 49 wherein the second layer further comprises a metalselected from the group consisting of Zn, Bi, Pb, and combinationsthereof.
 56. The coating according to claim 49 further comprising anintermediate layer wherein the intermediate layer is located between thefirst layer and the second layer.
 57. The coating according to claim 56wherein the intermediate layer is formed by electrolytic deposition. 58.The coating according to claim 56 wherein the intermediate layercomprises a metal selected from the group consisting of Cu, Ni, andcombinations thereof.
 59. The coating according to claim 49 furthercomprising a varnish layer which comprises a varnish wherein the secondlayer is located between the first layer and the varnish layer.